1. Field of the Invention
The present invention relates to biotechnology, and more specifically to a method for producing amino acids. The present invention more specifically relates to a method for producing L-homoserine, L-threonine, L-valine, or L-leucine using a bacterium belonging to the genus Escherichia. 
2. Brief Description of the Related Art
The present inventors obtained, with respect to E. coli K-12, a mutant having a mutation, thrR (herein referred to as rhtA23) that confers resistance to high concentrations of threonine or homoserine in a minimal medium (Astaurova, O. B. et al., Appl. Bioch. And Microbiol., 21, 611-616 (1985)). The mutation resulted in improved production of L-threonine (SU Patent No. 974817), homoserine, and glutamate (Astaurova, O. B. et al., Appl. Bioch. And Microbiol., 27, 556-561, 1991) by the respective E. coli producing strains.
Furthermore, the present inventors have revealed that the rhtA gene exists at 18 min on E. coli chromosome, and that the rhtA gene is identical to the ORF1 between the pexB and ompX genes. The unit expressing a protein encoded by the ORF has been designated the rhtA (rht: resistance to homoserine and threonine) gene. The rhtA gene includes a 5′-noncoding region which includes a SD sequence, ORF1, and a terminator. Also, the present inventors have found that a wild-type rhtA gene imparts resistance to threonine and homoserine if cloned in a multicopy state, and that the rhtA23 mutation is an A-for-G substitution at position −1 with respect to the ATG start codon (ABSTRACTS of 17th International Congress of Biochemistry and Molecular Biology in conjugation with 1997 Annual Meeting of the American Society for Biochemistry and Molecular Biology, San Francisco, Calif. Aug. 24-29, 1997, abstract No. 457).
During cloning of the rhtA gene, at least two different genes were found which impart threonine and homoserine resistance when in a multicopy state in E. coli. One is the rhtA gene, and the other gene was found to be the rhtB gene, which confers homoserine resistance (Russian Patent Application No. 98118425).